The present invention relates to a catalyst for a fuel cell, particularly a catalyst for a polymer solid electrolyte fuel cell, and a membrane electrode assembly and a fuel cell using the catalyst.
Solid polymer fuel cells, particularly methanol-type solid polymer fuel cells using a methanol solution as a fuel, can be operated at a low temperature and can realize a reduction in size and a reduction in weight and thus have recently drawn attention as a power supply, for example, for mobile equipment, and research and development thereof have been forwarded.
The performance of conventional fuel cells, however, is unsatisfactory for wide spreading. In fuel cells, chemical energy is converted to electric power by an electrode catalyst reaction, and high-activity catalysts are indispensable for the development of high-performance fuel cells.
Up to now, PtRu has generally been used as a catalyst for an anode electrode in fuel cells. The voltage loss by the PtRu catalyst is about 0.3 V with respect to the theoretical voltage 1.21 V of the electrode catalyst reaction, and anode catalysts having a higher activity (methanol oxidation activity) than the catalytic activity of the PtRu have been desired. From this viewpoint, in order to improve the methanol oxidation activity of the PtRu catalyst, various studies, for example, on the addition of other elements to the PtRu, have been made.
For example, U.S. Pat. No. 3,506,494 describes the effect attained by the addition of ten metals such as tungsten and tantalum, and JP-A 2006-278217 refers to the effect attained by the addition of silicon (Si), aluminum (Al), titanium (Ti) and the like. However, it should be noted that the reaction field in the catalyst reaction is present on the surface of nano-size catalyst particles, and, since several atomic layers on the catalyst surface substantially govern the catalyst activity, the surface state of the catalyst possibly varies depending upon the catalyst synthesis process even when the composition of the catalyst is identical. Up to now, solution methods such as immersion methods have generally been used for the catalyst synthesis. The solution methods, however, suffer from a problem that, for elements which are resistant to reduction and are hardly alloyed, the control of the catalyst surface is difficult.
On the other hand, the synthesis of catalysts by sputtering or vapor deposition is more advantageous than the solution method from the viewpoint of the control of the material. At the present time, however, the influence of a change in conditions such as the type of elements, catalyst composition, substrate material, and substrate temperature, on a catalyst production process has not been fully studied. Since the catalyst for fuel cells is nanoparticles, the surface electron state of catalyst particles and the nanostructure of the particles greatly depend upon the type and addition amount of elements added, and, thus, it is considered that, in order to realize high activity and high stability, the type of addition elements, the amount of elements added, and a combination of addition elements should be optimized. To this end, in PCT Publication No. 2005-532670, studies have been made on a production process by sputtering, and finding on elements other than Pt and Ru is described. Further, JP-A 2006-128118 discloses a catalyst comprising a compound, selected from silicon, aluminum, and titanium, added to a catalyst metal.
In all the above prior art techniques, studies on the influence of the catalyst composition on the catalyst activity are unsatisfactory, and the provision of an improved catalyst for a fuel cell, which is excellent in both methanol oxidation activity and catalyst stability, has been still desired.